The present invention relates to an electric working machine including electric motors, a movable speed adjustment member, a potentiometer for producing a voltage variable in response to the movement of the member, and a control unit for controlling the electric motors on the basis of the voltage to thereby change a velocity of the electric working machine.
Known electric working machines include electric motors and motor drivers for driving the motors. Most of the electric motors include controllers for controlling the motors through the motor drivers to prevent the motors from being placed under an overload. Such electric working machines are disclosed in, for example, Japanese Patent Kokai (Laid-Open) Publication No. HEI 9-284991 entitled xe2x80x9cMOTOR PROTECTOR FOR SELF-PROPELLED TRAVEL DEVICExe2x80x9d and Japanese Utility Model Laid-Open Publication No. HEI 5-70101 entitled xe2x80x9cOVERLOAD PREVENTING APPARATUS FOR UNMANNED CONVEYORxe2x80x9d.
The electric working machine disclosed in the publication No. HEI 9-284991 includes an electric motor, a detection circuit for detecting current flowing through the motor, an alarm lamp, and an alarm buzzer. When the current flowing through the motor exceeds a previously set value, the lamp flashes or otherwise is lit while the buzzer produces an alarm. If the current is maintained above the set value for a given period of time, the motor comes out of operation. Consequently, the working machine stops traveling.
The electric working machine disclosed in the publication No. HEI 5-70101 includes a controller, a traveling indicator, electric motors, and detectors for detecting speeds of the electric motors to provide signals indicative of the motor speeds. The traveling indicator provides a signal indicative of a previously set velocity of the motors. If the signal(s) which the detector(s) provide become greater in level than the signal from the traveling indicator in a short time, the controller judges the motor(s) to be placed under an overload to thereby bring the same out of operation. As a result, the working machine stops traveling.
The working machines as disclosed in the above publications are designed such that, when placed under the overload, the motors come out of operation for the purpose of protection. This is advantageous because electric components associated with the motors can be protected as well.
For some application, however, it may be preferred for the aforementioned working machine to rather keep traveling than stop traveling even if the motor is placed under the overload.
Japanese Patent Kokoku (Post-Exam) Publication No. SHO-48-4260 entitled xe2x80x9cCONTROLLER FOR PROPELLING ELECTRIC VEHICLE AND DETERMINING DIRECTION OF PROPULSION THEREOFxe2x80x9d discloses an electric vehicle including a controller. The controller includes an operational lever, batteries, potentiometers, and electric motors. The batteries apply voltages to the potentiometers. When the lever is manipulated, sliding contacts of the potentiometers are caused to move. The movement of the sliding contacts controls the electric motors. More specifically, a direction of rotation of the motor depends upon which side of a resistor of each potentiometer the sliding contact is positioned on. A velocity of the electric vehicle depends upon where the sliding contacts are positioned.
The velocity of the vehicle depends upon where the sliding contacts are positioned. Thus, how the velocity of the vehicle varies depends upon resistances of the resistors of the potentiometers.
For some application, however, the electric vehicle may be easy to operate if it provides its velocity variable in small increments or decrements when traveling fast or otherwise slowly. Contrarily, it may be often preferred for the electric vehicle to change in velocity in large increments or decrements when traveling fast or otherwise slowly. The electric vehicle is desirably changed in velocity in small increments or decrements especially when traveling within a range of velocity an operator often selects.
An object of the present invention is to provide an electric working machine including electric motors designed to keep operating even after coming out of an overload state.
A further object of the present invention is to provide an electric working machine designed to vary in velocity in small increments or decrements when traveling within a range of velocity an operator frequently selects.
According to one aspect of the present invention, there is provided an electric working machine comprising: drive wheels; electric motors for driving the drive wheels; a speed adjustment member movable within a range including a neutral range, a working low speed range, a working middle speed range, and a high speed traveling range; a potentiometer having a voltage applied thereto, the voltage varying in response to the movement of the speed adjustment member; a control unit for controlling the electric motors by means of motor drivers on the basis of the voltage applied to the potentiometer; and the electric working machine stopping traveling when the speed adjustment member is in the neutral range, the electric working machine traveling at a low velocity while performing a predetermined operation when the speed adjustment member is in the working low speed range, the electric working machine traveling at a middle velocity while performing the predetermined operation when the speed adjustment member is in the working middle speed range, the electric working machine traveling without performing the predetermined operation when the speed adjustment member is in the high speed traveling range.
The speed adjustment member may be rotational one having a pointer. In such a case, the member rotates to move the pointer within the respective ranges. Alternatively, the speed adjustment member may include plural buttons corresponding to the respective ranges.
In a preferred form of the present invention, the velocity of the electric working machine varies more gradually when the speed adjustment member moves in the working middle speed range than when the speed adjustment member moves in the working low speed range and the high speed traveling range.
The working middle speed range provides velocities that are more frequently chosen by the operator than other ranges. Moving the speed adjustment member within the working middle speed range varies the velocity of the electric working machine in small increments or decrements. This is advantageous in that the working machine performs the predetermined or required operation with increased precision.
In a further preferred form of the present invention, the control unit including: a stop section for issuing to the motor drivers a current carrying stop command for stopping carrying currents to the electric motors when current(s) flowing through the electric motor(s) exceed an upper limit threshold level; a restart section for issuing to the motor drivers a current carrying start command for carrying currents to the electric motors to prevent the current(s) flowing through the electric motor(s) from falling to zero after issuance of the current carrying stop command; and a repetition section for repeating the issuance of the current carrying stop command and the issuance of the current carrying start command.
Generally, after the carrying of current to an electric motor is stopped, under the influence of inductance of a coil of the motor, it takes a little time (about 1 m sec.) for the current flowing through the motor to drop to 0 (A). The present invention advantageously utilizes such an electric motor.
As is apparent from the foregoing description, after issuing the current carrying stop command with the motor(s) placed under an overload, the control section issues the current carrying start command to prevent the current (s) flowing through the motor(s) from dropping to 0 (A), such that the carrying of currents to the motors is restarted. With this arrangement, the stopping and restarting of the carry of the currents can alternate at short intervals. Since the control unit starts the carrying of currents to the motors while the current(s) are decreasing from the upper limit threshold level, the current(s) are varied by a limited amount after the motor(s) are brought to the overload state. Consequently, torques produced by the motors is varied to a limited extent. It thus becomes possible to operate the motors, bringing the motor(s) out of the overload state. This enables the snow removing machine to keep traveling.
With this arrangement, excessive current does not flow through the motors and their associated components to protect the motors and the components.